How to create a material which lets light pass through unaffected?
If you want to set this up per material, you can use the Light Path node to make a material transparent to shadow rays: (Note the Geometry node is not relevant to this first example)



with this result...



...but as you can see from the shadow on the floor plane, this is transparent in both directions. If you want to fix that, you can wire in the Geometry node, so the the surface is transparent only to shadow rays to which it is backfacing .. (not forgetting the rays are fired from the shadowed surface towards the light)...



.. the multiply is being used to 'AND' the two booleans.. with this result.



You can go further, to make the material transparent only to specified lights.. see here.. or here.. but as you can see, that gets more involved.